JP2014175533A - Laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique related to optimization of the waveguide cross-sectional shape for improving the net gain in a laser element.SOLUTION: A laser element has a waveguide including a resonance structure for resonating electromagnetic wave. The waveguide includes a gain medium 103, a first negative permittivity medium 104 of a negative permittivity real part in contact with the gain medium, a second negative permittivity medium 102 of a negative permittivity real part in contact with the gain medium, and a side face structure 105 on a positive permittivity real part in contact with the side face of the gain medium, and held by the first and second negative permittivity media. The waveguide includes a section where the width w between the side faces of the gain media 103 is equal to or less than two times of the thickness h of the side face structure 105 held by the first and second negative permittivity media.

Description

The present invention relates to a laser element. In particular, the present invention relates to a current injection type laser element in an electromagnetic wave (hereinafter also referred to as a terahertz wave or the like) in a frequency band in a frequency region from a millimeter wave band to a terahertz band (30 GHz to 30 THz). More specifically, for example, the present invention relates to a current injection type laser device including a waveguide through which surface plasmons propagate.

As a new kind of semiconductor laser oscillator, quantum cascade laser based on transition between energy levels (intersubband transition) of carriers in the same energy band of conduction band or valence band, oscillator which processed tunnel diode into waveguide, It has been known. Recently, there is also a demand for electromagnetic resources in the terahertz band that are considered useful for biological sensing and the like. Along with this, development has been carried out to make the oscillation wavelength longer in the quantum cascade laser, and development to make the oscillation frequency higher in the waveguide-shaped tunnel diode oscillator. Yes.

Non-Patent Document 1 discloses several methods for causing a quantum cascade laser to oscillate in the terahertz band. One of them is a metal-metal waveguide in which a semiconductor gain medium is sandwiched between metals. The metal functions as a negative dielectric constant medium in which the real part of the dielectric constant is negative in this frequency band. At this time, the waveguide mode guided to the cladding of the negative dielectric constant medium is an electromagnetic wave contributed by polarization oscillation of charge carriers in the negative dielectric constant medium called surface plasmon. Since surface plasmons have no diffraction limit, much of the mode intensity can be confined in the gain medium. By using such a method, laser oscillation with an oscillation frequency of 1.2 THz (oscillation wavelength λ = 250 μm) is achieved.

Patent Document 1 discloses that a semiconductor gain medium and a dielectric having a side structure are a negative dielectric constant medium having a negative dielectric constant in a millimeter wave band to a terahertz band (a constituent material is a metal or a highly doped semiconductor). ) Is disclosed. At this time as well, the waveguide mode guided to the cladding of the negative dielectric constant medium is surface plasmon. However, by introducing a dielectric side surface structure, it is possible to effectively reduce the waveguide loss. By using such a technique, for example, a waveguide oscillator having an oscillation frequency of 0.3 THz (oscillation wavelength λ = 1000 μm) that can oscillate even with a tunnel diode or the like is achieved.

Japanese Patent No.48557027

Benjamin S. Williams, Nat. Photonics, Vol. 1 (2007), 97

However, further improvements in the characteristics of laser elements from the millimeter wave band to the terahertz band have been demanded, and improvement of the waveguide cross-sectional shape as one element has been required. However, in Non-Patent Document 1, there is no description about the width direction of the waveguide, and even in Patent Document 1 in which the width direction is taken into account, the width of the waveguide is only disclosed to be equal to or less than the oscillation wavelength. Therefore, the width of the waveguide for improving the net gain (= the difference obtained by subtracting the waveguide loss from the gain) in the conventional laser oscillator has not been well known. The present invention has been made in view of such problems, and the object thereof is a technique for optimizing the cross-sectional shape of a waveguide in an element such as a laser element in a frequency band in a frequency region from a conventional millimeter wave band to a terahertz band. Is to provide.

The laser element of the present invention is a laser element having a waveguide including a resonance structure for resonating an electromagnetic wave, and the waveguide is in electrical contact with a gain medium for generating the electromagnetic wave and the gain medium. A first negative dielectric constant medium having a negative real part of the dielectric constant with respect to the electromagnetic wave provided in contact with the gain medium, and the first negative dielectric constant medium via the gain medium. A second negative dielectric constant medium having a negative dielectric constant relative to the electromagnetic wave provided on the opposite side, and a side surface of the gain medium, the first and second negative dielectric constant media being A side structure in which a real part of a dielectric constant for the sandwiched electromagnetic wave is positive. The waveguide includes a section in which the width w between the side surfaces of the gain medium is equal to or less than twice the thickness h sandwiched between the first and second negative dielectric constant media of the side structure.

According to the present invention, in the conventional laser element, it is possible to expand the frequency band where the net gain exists, that is, the frequency band where the gain exceeds the waveguide loss. For example, in the case of a quantum cascade laser, the oscillation wavelength band can be further expanded, and in a waveguide-shaped tunnel diode oscillator, the oscillation frequency band can be further increased.

Sectional drawing which shows the structure of the laser element which concerns on 1st embodiment. Sectional drawing which shows the structure of the laser element which concerns on the modification of 1st embodiment. Sectional drawing which shows the structure of the laser element which concerns on 2nd embodiment. Sectional drawing which shows the structure of the laser element which concerns on 3rd embodiment. The top view which shows the structure of the laser element which concerns on 4th embodiment. Sectional drawing which shows the structure of the laser element which concerns on Example 1, and the graph which shows the calculation result of waveguide loss (alpha) and the wave number (beta) of a propagation direction.

The laser element of the present invention is provided with a section in which the width w between the side surfaces of the gain medium of the waveguide is not more than twice the thickness h sandwiched between the first and second negative dielectric constant media of the side structure. Features. According to the present invention, the waveguide cross-sectional shape can be optimized to improve the net gain required for the conventional laser element.

The problem of the waveguide cross-sectional shape can be examined in terms of an electric circuit by replacing the electromagnetic wave gain in the gain medium of the conventional laser element with a negative differential conductance Gd (<0). For quantum cascade lasers, gain can also be expressed as negative optical conductivity σ (λ) as a function of wavelength λ. Gd (λ) and σ (λ) are in a proportional relationship. In the case of a tunnel diode, the negative conductance Gd in the frequency region from DC (direct current) to around the terahertz band does not change so much, so it is only necessary to extend Gd in DC from the millimeter wave band to the terahertz band. Such replacement is a good approximation especially when the surface plasmon mode held in the waveguide is a single mode.

When the width of a waveguide that propagates a single mode is relatively narrow, there is a relationship of electromagnetic wave gain ∝−Gd through a proportional coefficient. Similarly, the waveguide loss is decomposed into the electrical resistance Rs in the longitudinal direction per unit length of the waveguide and the conductance Gp in the thickness direction per unit length of the waveguide through the proportionality constant. The waveguide loss α at this time can be described as the first approximation from the distributed constant circuit theory as follows.
α ＝ Rs / Zc + GpZc
Here, Zc is the characteristic impedance of the waveguide, and is proportional to the ratio of the electric field to the magnetic field of the electromagnetic wave propagating through the waveguide. Since the negative dielectric constant medium in the waveguide of the conventional laser element is made of a material having a relatively large electrical conductivity such as a metal or a highly doped semiconductor, the second term on the right side is Gp to Gd (<0). You can think about it. This term is negative. α has a unit of m ⁻¹ , and when the absolute value of the second term on the right side exceeds the absolute value of the first term on the right side, α means negative, that is, a net electromagnetic wave gain. Rs is determined by the conductor loss of the negative dielectric constant medium, but Zc can be adjusted by the waveguide cross-sectional shape. Therefore, there is room for optimization in this part.

In another expression, the characteristic impedance Zc can be described as follows using the inductance Ls in the longitudinal direction per unit length of the waveguide and the capacitance Cp in the thickness direction per unit length of the waveguide.
Zc = √ (Ls / Cp)
In the conventional laser element, Ls is inversely proportional to the waveguide width w, and Cp is proportional to the waveguide width w, and thus is Zc∝1 / w (meaning that ∝ is proportional).

Similarly, the elements of the waveguide loss α are arranged as follows depending on the waveguide width w dependency.
Case 1) The gain medium is sandwiched between negative dielectric constant media without a side structure.
Rs∝1 / w
Rs / Zc∝const. (The first term on the right side is a constant regardless of w)
Gd∝w
GdZc∝const. (The second term on the right side is a constant regardless of w)
As described above, the merit of scale for optimizing the waveguide width w is hardly considered in the first approximation.

On the other hand, Case 2) The gain medium and the side structure are sandwiched between negative dielectric constant media.
Rs = const.
Rs / Zc∝w (The first term on the right side is proportional to w)
Gd∝w
GdZc∝const. (The second term on the right side is a constant regardless of w)
As described above, the narrower the waveguide width w, the smaller the Rs / Zc, there is a merit of scale, and the net electromagnetic wave gain can be increased. More precisely, the w dependence of Rs varies slightly depending on the detailed structure, but it is only less dependent than the first order of w (w dependence approaches zero but does not become a constant). , Resulting in the same conclusion. The present invention optimizes the cross-sectional shape of the waveguide based on such qualitative examination in terms of electric circuit, and includes the following embodiments.

(First embodiment)
The laser element according to the first embodiment will be described with reference to FIG. FIG. 1 is a schematic diagram showing a cross section of a waveguide according to the present embodiment, and the waveguide extends along the z direction (direction perpendicular to the paper surface). The z direction indicates the longitudinal direction of the waveguide and the propagation direction of the electromagnetic wave. The x direction corresponds to the width direction of the waveguide, and the y direction corresponds to the thickness direction of the waveguide.

In the first embodiment, reference numerals 101 and 102 denote negative dielectric constant media in an oscillation frequency band in a frequency region from the millimeter wave band to the terahertz band. It consists of a metal, a heavily doped semiconductor, or both. Reference numeral 103 denotes a gain medium that generates electromagnetic waves in the same frequency band. The negative dielectric constant media 101 and 102 are arranged on opposite sides of the gain medium 103. Typically, a semiconductor multilayer structure is employed in which current injection is performed and gain is obtained. The gain medium 103 is sandwiched between the negative dielectric constant media 101 and 102 and is also in electrical contact so that current can be injected into the gain medium 103 through the negative dielectric constant media 101 and 102. A voltage supplied by an external electric field applying means (not shown) is applied above and below the gain medium 103 via the negative dielectric constant media 101 and 102, and thus current can be injected into the gain medium 103. Each of the negative dielectric constant media 101 and 102 is a cladding of a first negative dielectric constant medium and a cladding of a second negative dielectric constant medium, and a surface plasmon mode can propagate in the z direction in the waveguide. Reference numeral 105 denotes a positive dielectric constant medium having a positive real part of dielectric constant. It may be composed of a dielectric, or it may be composed of air, that is, an air bridge. The positive dielectric constant medium 105 forms a side structure adjacent to the side face of the gain medium 103 and is similarly sandwiched between the negative dielectric constant media 101 and 102.

At least one of the negative dielectric constant media 101 and 102 of the present embodiment includes a rib shape 104 that protrudes toward the gain medium 103 in a portion extending over the width of the gain medium 103. In such a waveguide cross-sectional shape, the width w of the waveguide is defined by the width from one side surface of the gain medium 103 in the x direction to the other side surface. The height in the y direction of the positive dielectric constant medium 105 having a side structure sandwiched between the negative dielectric constant media 101 and 102 is defined as h.

In the case of this embodiment, since the electrical resistance Rs in the longitudinal direction of the waveguide is determined by a wide portion in the width direction of the negative dielectric constant media 101 and 102, it can be said that Rs = const. The negative differential conductance Gd proportional to the electromagnetic wave gain of the gain medium is determined by the width w of the waveguide and is Gd∝w. In the case of the present embodiment, since the magnetic field lines of electromagnetic waves leak to the side structure, the longitudinal inductance Ls of the waveguide is rather determined by the height h of the side structure, and the dependence on the waveguide width w is relatively small. Become. The capacitance Cp in the thickness direction per waveguide unit length is determined by the waveguide width w because the gain medium 103 is a semiconductor having a relatively high dielectric constant. Therefore, the characteristic impedance Zc is roughly considered to be Zc∝√ (h / w). Here again, the waveguide loss α = Rs / Zc + GpZc is rearranged as follows.
Rs = const.
Rs / Zc∝√ (w / h)
Gd∝w
GdZc∝√ (h / w)
Considering the height h of the side structure, the structure dependence is different from the previous discussion, but as a result, the narrower the waveguide width w, the smaller Rs / Zc and the larger GdZc. The gain can be increased. That is, the positive Rs / Zc decreases and the absolute value of negative GdZc increases and the waveguide loss α decreases (positive α decreases or the absolute value of negative α increases). The electromagnetic wave gain increases. When the height h of the side structure is defined as in the present embodiment, it is preferable that the waveguide width w be equal to or smaller than the side structure height h because the net gain can be effectively increased. Speaking of a wider tolerance, the waveguide width w is preferably not more than twice the side structure height h. That is, in the waveguide, the width w between the side surfaces of the gain medium is a section of the thickness h or less sandwiched between the first and second negative dielectric constant media of the side structure, or a section of twice or less the thickness h. Is provided. The waveguide may have a width w that is less than or equal to thickness h or less than or equal to twice the thickness h over the entire length of the resonant structure.

As a modification of the first embodiment, a waveguide cross section as shown in FIG. 2 is also possible. In the above discussion, the position of the gain medium 103 in the thickness direction in the waveguide is arbitrary. For example, as shown in FIG. 2A, the gain medium 203 may be eccentric to the second negative dielectric constant medium 202 side, or conversely, may be eccentric to the first negative dielectric constant medium 201 side. Furthermore, the rib shape of the cladding of the negative dielectric constant medium is also arbitrary. This is a discussion on a scale smaller than the degree of leakage of magnetic lines of force from the gain medium 203 toward the side structure 205, but it does not have to be a strip shape as shown in FIG. 2A. For example, FIG. The trapezoidal rib 204 as shown in FIG. That is, the shape of the rib-shaped edge of the clad within the range in which the magnetic field lines are leaking is arbitrary.

(Second embodiment)
A laser element according to the second embodiment will be described with reference to FIG. FIG. 3 is a schematic diagram showing a cross section of the waveguide according to the present embodiment. Like the first embodiment, the waveguide extends along the z direction.

The present embodiment shows an example in which the first embodiment is configured more practically. Reference numerals 311 and 312 denote negative dielectric constant media in contact with the gain medium 303, and a highly doped semiconductor is selected. Reference numerals 301 and 302 denote negative dielectric constant media in contact with the heavily doped semiconductors 311 and 312, and a metal is selected. There are two reasons for this. This is because the height h of the side structure 305 can be set large and the degree of freedom in selecting the waveguide width w (= or <h or = or <2h) is increased. The gain medium 303 typically employs a semiconductor multilayer structure such as a quantum cascade laser, a tunnel diode, or a resonant tunnel diode. Since the negative dielectric constant media 311 and 312 are highly doped semiconductors, the laminated structure of the negative dielectric constant medium 311, the gain medium 303, and the negative dielectric constant medium 312 can be easily formed by means such as continuous film formation. I can do it. The same width w of the negative dielectric constant medium 311, the gain medium 303, and the negative dielectric constant medium 312 can be easily formed by dry etching or wet etching because semiconductors having substantially the same selection ratio can be employed. However, a metal crimping process is usually required to realize such a structure.

What is important in the present embodiment is to realize the above-described constant Rs (electric resistance in the longitudinal direction) on the structure. For that purpose, it is necessary to permeate electromagnetic waves into wide portions of the negative dielectric constant media 301 and 302 in the width direction. In the oscillation frequency band in the present embodiment, the metal has a thin skin depth, and the semiconductor has a thick skin depth. If such a property is used, it is easier to realize a constant Rs if a semiconductor is used for the ribs 311 and 312 of the negative dielectric constant medium. In order to make the height h of the side structure 305 large, a semiconductor in which the ribs 311 and 312 are highly doped is used. Typically, a carrier concentration of about 1 × 10 ¹⁹ cm ⁻³ should be selected.

In the present embodiment, two electrodes 321 and 322 for injecting current into the gain medium 303 through the metal negative dielectric constant media 301 and 302 are provided. The laser oscillation operation of the element of this embodiment is achieved by connecting a voltage source (not shown) to the two electrodes 321 and 322.

(Third embodiment)
A laser element according to the third embodiment will be described with reference to FIG. FIG. 4 is a schematic diagram showing a cross section of the waveguide according to the present embodiment. Like the first embodiment, the waveguide extends along the z direction.

The present embodiment shows an example in which the first embodiment is configured more practically. Reference numeral 400 denotes a conductive semiconductor substrate. For example, the carrier concentration is preferably 1 × 10 ¹⁸ cm ⁻³ or more. Further, the negative dielectric constant medium 401 uses a highly doped semiconductor. The semiconductor layer 401 is desirably thicker than the skin depth in the oscillation frequency band of the present embodiment, and is preferably a semiconductor having a carrier concentration of 1 × 10 ²⁰ cm ⁻³ having a thickness of several μm (for example, 2 μm and 3 μm), for example. The metal 421 also serves as a cladding and an electrode for a negative dielectric constant medium. Such a configuration is one of the preferable configurations that can facilitate manufacture while reducing leakage of electric lines of force downward from the gain medium 403. The metal 402, the semiconductors 411 and 412 doped with high concentration carriers, the gain medium 403, and the electrode 422 are the same as in the second embodiment. As the positive dielectric constant medium 405, BCB (benzocyclobutene) which is a dielectric having a relatively low loss and a low dielectric constant in a frequency region from the millimeter wave band to the terahertz band may be used.

In the present embodiment, the substrate 400, the negative dielectric constant media 401 and 411, the gain medium 403, and the negative dielectric constant medium 412 (portions that are in electrical contact with the gain medium of the second negative dielectric constant medium) are all semiconductors. Such a laminated structure on the semiconductor substrate 400 can be easily formed by using a semiconductor heteroepitaxial film formation technique. Furthermore, no metal crimping process is required.

(Fourth embodiment)
A laser element according to the fourth embodiment will be described with reference to FIG. FIG. 5 is a schematic view showing the upper surface of the waveguide according to this embodiment. The waveguide extends along the z direction, and its end face is cut.

The present embodiment shows an example of a configuration along the propagation direction of the surface plasmon of the first embodiment, and the laser resonator is a Fabry-Perot resonator sandwiched between end faces formed by cutting a waveguide. It is constituted by. The electromagnetic wave is made a standing wave by utilizing reflection from the end face. That is, the waveguide has a resonance structure having at least two end faces in the propagation direction of the electromagnetic wave, and uses the reflection from the end face to make the electromagnetic wave a standing wave. The gain medium 503a and the negative dielectric constant medium rib 504a which are drawn to overlap in the top view have the same cross-sectional shape as in the first embodiment. Reference numerals 506 and 507 denote end faces. The length L from the end face 506 to the other end face 507 is equal to L, which is an integral multiple of π / β, as known in semiconductor laser technology, where β is the wave number in the propagation direction of the surface plasmon mode. This makes it possible to select the oscillation wavelength. Since the factor of an integer multiple is typically about 1 to 100 times, the typical length L is about several tens of μm to several mm.

Also in this embodiment, when the height h of the side structure is defined, it is preferable that the waveguide width w is equal to or less than the side structure height h because of the net gain −α. FIG. 5A shows an example in which the waveguide width w is constant along the propagation direction, that is, the z direction. In this example, the net gain −α is large over the entire section in the propagation direction of the waveguide. However, the narrow waveguide width does not contribute to improving the output of the laser oscillator. For this reason, the waveguide width w (z) is a function of the propagation direction z as shown in FIG. 5B showing the gain medium 503b and the rib 504b of the negative dielectric constant medium which are drawn to overlap each other in the top view. The structure which becomes is also considered. The net gain -α is large in the interval of w (z) = or <h (or = or <2h), and the current injection can be increased in the interval of w (z)> h (or> 2h). Such a structure is convenient as a structure that can optimize the gain and the oscillator output. The metal 502 is the same as in the second embodiment.

More specific laser elements will be described in the following examples.
Example 1
A more specific example 1 corresponding to the third embodiment will be described. The laser element according to this example will be described with reference to FIG. In addition to the schematic diagram 6A showing the cross section of the waveguide, in this embodiment, the electric circuit calculation is also performed as shown in FIGS. 6B and 6C, and the above-described qualitative discussion is made concrete.

In FIG. 6A, reference numeral 600 denotes a substrate. As the gain medium 603, an InGaAs / InAlAs multiple quantum well lattice-matched to the InP substrate is selected. For example, a resonant tunnel diode composed of a semiconductor multilayer structure of 5.0 / 1.3 / 5.6 / 2.6 / 7.6 / 1.3 / 5.0 in the −y direction is selected. Here, the number is the thickness of each part in the unit of nm, the part without an underline is a well made of InGaAs, and the underlined part is a potential barrier made of InAlAs. These layers are undoped without intentional carrier doping. In the negative dielectric constant medium 611, for example, an n-InGaAs semiconductor film (thickness 50 nm) is used for electrical contact with the resonant tunneling diode 603. Most of the ribs 611 use an n-InGaAs semiconductor film (thickness: 440 nm) having an electron concentration of 1 × 10 ¹⁹ cm ⁻³ in order to reduce the conductor loss. The negative dielectric constant medium 612 is similar to the negative dielectric constant medium 611.

In this embodiment, the negative dielectric constant medium 601 uses an Au thin film (thickness: 500 nm) for reducing the conductor loss. Therefore, the InP substrate in the InGaAs / InAlAs multiple quantum well on the InP substrate has already been removed. Such a structure is manufactured using an Au pressure bonding process. Specifically, Au 601 (thickness 250 nm) formed on an InGaAs / InAlAs multiple quantum well and Au 601 formed on a conductive Si substrate 600 are used. (Thickness 250 nm) can be produced by pressure bonding. For removing the InP substrate, wet etching such as hydrochloric acid may be used.

Thereafter, wet etching is performed on the semiconductor portions 611, 603, and 612, and the width of the waveguide is processed to w = 1 μm. BCB is used for the side structure 605 of the gain medium 603. The thickness of BCB is selected as h = 1μm. Finally, if an Au thin film (thickness 500 nm) is formed as the negative dielectric constant medium 602 on the n-InGaAs 612 and the BCB 605, the waveguide cross-sectional structure is completed.

In the present embodiment, when a bias of 0.8 V is applied between the Au 602 serving as the cladding and electrode of the negative dielectric constant medium and the back electrode 621, the gain obtained by the resonant tunneling diode 603 is in the frequency band near 1 THz from DC. It is calculated to be about 700cm- ¹ . This is Gd = -12 mS / μm ² in terms of negative differential conductance. FIG. 6B shows the result of the electric circuit calculation of the waveguide loss α in consideration of such parameters. In the figure, the net gain is obtained in the negative region on the vertical axis. That is, it was found that a net gain was obtained from around DC to around 1.6 THz. For reference, the same configuration was compared with the case of w = 5 μm and h = 2 μm. It can be seen that by designing w / h to be small, the oscillatable region is expanded and oscillation is possible even at a frequency higher than 0.5 THz, which was impossible to oscillate in the configuration of w = 5 μm and h = 2 μm.

The laser resonator of this example uses the Fabry-Perot resonator of the fourth embodiment for simplicity. Of course, a distributed feedback type DFB resonator may be used as is well known in the semiconductor laser technology. Furthermore, in this example, 0.8 THz was selected as the oscillation frequency because colon cancer sensing was performed using a terahertz laser oscillator. This is because 0.8THz is useful in cancer diagnosis because the contrast between colorectal cancer and a normal site is clear. FIG. 6C shows the result of electric circuit calculation of the wave number magnitude β in the propagation direction of the surface plasmon mode in consideration of the same parameters as in FIG. 6B. Since the wave number magnitude β at the oscillation frequency of 0.8 THz was calculated to be 750 cm ⁻¹ from the figure, the length of the waveguide was selected to be L = 42 μm, which is one time of π / β in this embodiment. Of course, L which is an integral multiple of this length may be selected. Even if the end face may be cleaved or processed using dry etching, sufficient surface accuracy is secured for electromagnetic waves from the millimeter wave band to the terahertz band (electromagnetic waves including part of the frequency range of 30 GHz to 30 THz). it can.

Another feature of the present invention is that β can be made relatively small as in this embodiment by designing w / h to be small. In other words, since L can be designed relatively large, it is convenient to increase the output of the laser oscillator. Furthermore, by designing w / h to be small, the efficiency of extracting electromagnetic waves from the end face of the Fabry-Perot resonator to free space is improved. For these reasons, the waveguide cross-sectional shape of w / h = or <1 is very preferable for laser elements from the millimeter wave band to the terahertz band.

When the gain medium is a tunnel diode as in this embodiment, the frequency band with electromagnetic wave gain extends from DC to the high frequency side, but in the case of a quantum cascade laser, the frequency band with electromagnetic wave gain is several THz. The center is a Lorentz function type with a width of sub-terahertz. When this embodiment is applied to a laser element having a gain medium of a quantum cascade laser, the net gain is expanded to its high frequency side and low frequency side. In this example, w / h is set to 1 or less, but for cancer diagnosis, even if w / h is 2 or less (that is, width w is less than twice the thickness h), the oscillation wavelength It is sufficient to realize 0.8 THz (λ = 375 μm).

The laser element according to the present invention is expected to be applied as an element device that can be used for manufacturing management, medical image diagnosis, safety management, and the like.

DESCRIPTION OF SYMBOLS 101 ... 1st negative dielectric constant medium, 102 ... 2nd negative dielectric constant medium, 103 ... Gain medium, 104 ... Rib shape of negative dielectric constant medium (1st negative dielectric constant medium ), 105 ... Side structure (positive dielectric constant medium)

Claims (9)

A laser element having a waveguide including a resonance structure for resonating electromagnetic waves,
The waveguide is
A gain medium for generating electromagnetic waves;
A first negative dielectric constant medium having a negative real dielectric constant for the electromagnetic wave provided in electrical contact with the gain medium;
A second negative dielectric constant provided in electrical contact with the gain medium and having a negative real part of the dielectric constant for the electromagnetic wave provided on the opposite side of the first negative dielectric constant medium via the gain medium; Medium,
A side surface structure that is provided in contact with the side surface of the gain medium and that has a positive real part of the dielectric constant for the electromagnetic wave sandwiched between the first and second negative dielectric constant media;
With
A laser element comprising a width w between side surfaces of the gain medium having a section equal to or less than twice a thickness h sandwiched between the first and second negative dielectric constant media of the side structure. The waveguide is
2. The laser according to claim 1, wherein a width w between side surfaces of the gain medium includes a section having a thickness h or less sandwiched between the first and second negative dielectric constant media of the side structure. element. The waveguide is
3. The laser device according to claim 1, wherein the width w of the gain medium is not more than twice the thickness h of the side structure over the entire length of the resonant structure. The waveguide is
4. The laser element according to claim 3, wherein a width w of the gain medium is equal to or less than a thickness h of the side structure over the entire length of the resonant structure. The waveguide is
5. The structure according to claim 1, wherein the resonance structure is configured to include at least two end surfaces in a propagation direction of the electromagnetic wave, and the electromagnetic wave is made a standing wave by using reflection from the end surface. 2. The laser element according to item 1. 6. The laser element according to claim 1, wherein the gain medium is a resonant tunnel diode or a quantum cascade laser. The first and second negative dielectric constant media are each composed of a heavily doped semiconductor in contact with the gain medium and a metal in contact with the semiconductor. The laser device according to Item. The portion of the first negative dielectric constant medium, the gain medium, and the second negative dielectric constant medium that are in electrical contact with the gain medium are all semiconductors. 2. The laser element according to item 1. 9. The laser element according to claim 1, wherein the frequency of the electromagnetic wave includes a part of a frequency region of 30 GHz to 30 THz.

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